1. Field of the Invention
The present invention relates to robotics, and, in particular, to a semi-active compliance device for controlling the position, force, and orientation of a package.
2. Statement of Related Art
In the field of package shipping, it is desirable to use robots to stack packages one on top of another for shipment. A package-stacking robot must be able to grab a package, lift it, move it to a desired position, and place it upon a stack in a desired orientation. Such a robot must be strong enough to lift and move a heavy package. Such a robot must also be gentle enough to place such a heavy package on a stack without knocking over any of the existing stacks or damaging any of the packages.
In addition, the nesting of packages in stacks should be compact to utilize space efficiently and to provide stable stacks. Errors in detecting and/or determining the position of the stack and inaccuracies in the positioning of the package by the robot will cause package nesting problems. One way to control the position and/or orientation of a package and the force and/or moment with which the package is moved when placing the package upon a stack is to use a robot with force sensors under force control. These force sensors generate signals representative of the magnitude and direction of external forces exerted onto the package, such as those that occur when the package bumps into a wall or another package. These force-sensor signals can be used for closed-loop force control of the motion of the package. The technology of force control requires very sophisticated force sensors and relatively complicated calculations which, in turn, reduces system speed and incurs high cost. To reduce the cost and improve system speed, remote center compliance (RCC) devices are conventionally used.
FIGS. 1 and 2 depict two different situations in which a robot (not shown) must adjust the orientation of a package 100 for proper placement upon a stack. In FIG. 1, package 100 must be rotated clockwise to be aligned with walls 102 and 104, while, in FIG. 2, package 100 must be rotated counter-clockwise to be properly oriented. As shown in FIG. 1, when the robot drives package 100 into wall 102, wall 102 exerts force F.sub.r in the positive X direction at corner 106 of package 100. As shown in FIG. 2, when the robot drives package 100 into wall 102, wall 102 exerts force F.sub.r in the positive X direction at corner 108 of package 100. When the force sensors (not shown) are located in the robot, the control algorithm cannot distinguish between the two situations depicted in FIGS. 1 and 2.
In applications other than package stacking, the problem depicted in FIGS. 1 and 2 may be handled by using a conventional remote center compliance (RCC) device such as those disclosed in U.S. Pat. Nos. 4,439,926 (Whitney et al.), 4,477,975 (De Fazio et al.), and 4,848,757 (De Fazio). These conventional RCC devices, however, are not well suited for package-stacking in the field of package shipping. The application of these conventional RCC devices is limited to small motion adjustment and insertion assemblies such as peg-hole devices. Moreover, since these conventional RCC devices are flexible, there will be unpredictability in controlling the remote center of heavy packages. This flexibility will also decrease the natural frequency of the robot system, making accurate control difficult.